The present invention relates to an optical recording medium, in which a substrate is bonded to a light transparent cover layer smaller in thickness than the substrate, one or more recording layer is provided between the substrate and the light transparent cover layer, and light is emitted from a light source by using an object lens from the side of the light transparent cover layer so as to record and reproduce information.
An optical information recording method has a variety of advantages including (1) recording and reproducing without making contact, (2) a recording density higher than a magnetic recording method by more than one digit, and (3) applicability to all the memory types of a Read-only type, a Write Once Read Many type, and a Rewritable type. The method also realizes a large-capacity file at low cost. Thus, a wide range of uses has been considered from industrial use to commercial use.
An optical recording medium such as an optical disk is adopted for this optical information recording method. A compact disk (CD) used as a music disk for reproduction and a mini disk (MD) for recording and reproduction have widely prevailed in the market. Such CD and MD have a light transparent substrate with thickness of 1.2 mm. One of the surfaces of the light transparent substrate includes an information recording layer and a protecting layer for protecting the information recording layer. In such CD and MD, light with a wavelength of 780 nm is emitted from the opposite side of the information recording layer to the light transparent substrate via an object lens having an NA (numerical aperture)=0.45 so as to record or reproduce information.
Incidentally, there has been demand for storing a large amount of information (data) such as moving picture information in the optical disk. Thus, a higher recording density has been considered. In the case of the optical disk, the recording density generally depends upon a spot size of a light beam on the optical disk. The spot size increases proportionally to xcex/NA (xcex represents a wavelength of a light beam, and NA represents a numerical aperture of an optical system); thus, a shorter wavelength of a light beam and a larger numerical aperture considerably contribute to a higher recording density.
However, a coma caused by the tilted optical disk increases proportionally to the cube of the NA, so that a larger numerical aperture of the optical system results in an extremely small tilt margin (allowable tilt angle) of the optical disk. Therefore, a slight tilt of the optical disk causes a blurred spot of the light beam, so that a high recording density cannot be achieved.
When the optical disk has the light transparent substrate with a thickness of t, the coma increases proportionally to txc2x7NA3. Regarding a digital versatile disk (DVD), an optical system is used with a large numerical aperture of NA=0.6, and a wavelength of a light beam is shortened from 780 nm to 650 nm so as to achieve a higher recording density. A thickness of the light transparent substrate is set at 0.6 (mm), which is smaller than those of the CD and the MD, so as to obtain a sufficient tilt margin even in the case of the optical system having a large numerical aperture of NA=0.6. Such a high recording density can achieve a recording capacity of 4.7 GB, which is equivalent to no less than 2 recording hours of moving picture signals.
However, the demand for a larger capacity and a longer recording time has been further increasing. In order to respond the aforementioned problem, an optical recording medium for storing data of 8 GB per one side is disclosed in Japanese Published Unexamined Patent Publication No. 302310/1998 (Tokukaihei 10-302310).
In the invention of the publication, a numerical aperture NA of an optical system is changed from 0.6 to 0.78 so as to achieve a high recording density. However, as described above, in the case of a larger numerical aperture of the optical system, a tilt margin (allowable tilt angle) enough for suppressing a coma becomes strict (smaller). The coma is caused by tilt of the optical disk. Therefore, in this invention, a thickness of the light transparent substrate is set smaller so as to minimize the likelihood of a coma caused by a tilt angle.
In this way, when the NA is increased to about 0.8, the thickness of the light transparent substrate needs to be reduced to about 0.1 mm. However, when the light transparent substrate is smaller in thickness, rigidity cannot be maintained by the light transparent substrate alone. It is therefore necessary to provide a reinforcing substrate on the back of the light transparent substrate with reference to a light source.
Hence, the optical disk used for the optical system with a large NA has a construction in which the thick substrate with high rigidity and a thin light transparent cover layer are bonded to each other, and the optical disk further includes a recording layer between the substrate and the light transparent cover layer. FIG. 11 shows a sectional drawing showing the construction of the optical disk. In this optical disk, a substrate 101 is used, which is made of a thermoplastic resin and includes guide concave 110 transferred upon molding. A recording layer 3 is formed on the substrate 101 by sputtering, and a light transparent cover layer 102 is bonded onto the recording layer 103 via an ultraviolet curing resin 104.
Further, in a pickup optical system having a large numerical aperture of about NA=0.8, a focal length of a lens is shorter, resulting in a short distance of approximately 0.3 mm between the lens and a surface of an optical disk (working distance). When the working distance is short, it is necessary to consider the likelihood of an accidental collision between the optical disk and the lens and to select a material with a certain degree of hardness for the light transparent cover layer.
However, when the hard light transparent cover layer is used, the two hard layers, that differ in rigidity, are bonded to each other via an adhesive; thus, when an impact is applied to a bonding part, the light transparent cover layer is likely to be severely deformed and peeled off.
Furthermore, the bonding part of the optical disk is exposed out of an outer curved surface of the optical disk, so that the outer curved surface is likely to receive an impact upon handling the optical disk; consequently, the light transparent cover layer tends to be peeled off at a bonding surface.
Moreover, the bonding part of the optical disk is exposed at a center hole of the disk as well, so that the bonding part is likely to receive an impact upon mounting the disk into a drive; thus, the light transparent cover layer tends to be peeled off at the bonding surface.
The present invention is devised to solve the aforementioned problem. The objective is to provide an optical recording medium, in which a substrate and a light transparent layer smaller in thickness than the substrate are bonded to each other, characterized in that an outer edge and an inner edge, on which exfoliation is likely to occur, are protected so as to secure reliability against exfoliation.
In order to achieve the above objective, an optical disk of the present invention, which includes a substrate, a light transparent cover layer bonded to the substrate with a smaller thickness than the substrate, and a first recording layer formed between the substrate and the light transparent cover layer, is characterized in that the light transparent cover layer is smaller in outer diameter than the substrate.
Further, it is desirable that the substrate and the light transparent cover layer be virtually formed into disks with center holes, the light transparent cover layer be smaller in outer diameter than the substrate, and the light transparent cover layer be larger in inner radius than the substrate.
According to the above arrangement, a bonding part of the light transparent cover layer and the substrate is not exposed out of outer and inner curved surfaces (circular cylinder surfaces), so that an impact is not directly applied to the bonding part; thus, the light transparent cover layer is less likely to be peeled off.
Moreover, the substrate is composed of an outer edge at a circumference, an inner edge around the center hole, and a between-edge part having a virtually flat surface. At least one of the outer edge and the inner edge is larger in thickness than the between-edge part. When the outer edge has an inner radius of r0 and the inner edge has an outer radius of r1, an outer radius of the light transparent cover layer is smaller than r0 and an inner radius of the light transparent cover layer is larger than r1.
According to the above arrangement, the bonding part of the light transparent cover layer and the substrate is protected by the outer edge and the inner edge that are provided at the outer and inner curved surfaces of the disk, so that an impact is not directly applied to the bonding part; thus, the light transparent cover layer is less likely to be peeled off.
Additionally, it is preferable to further provide a second recording layer between the first recording layer and the light transparent cover layer. Information is read out in the first recording layer by emitting light onto the first recording layer from the side of the substrate, and information is read out in the second recording layer by emitting light to the second recording layer from the side of the light transparent cover layer.
According to the above arrangement, regarding a bonded disk, in which the light transparent cover layer is less likely to be peeled off, compatibility between CD/DVD can be achieved with ease.
Furthermore, in order to achieve the aforementioned objective, the optical disk of the present invention, which includes a substrate, a light transparent cover layer bonded to the substrate with a smaller thickness than the substrate, and a first recording layer formed between the substrate and the light transparent cover layer, is characterized in that the substrate and the light transparent cover layer are formed into disks with center holes, and an outer curved surface (outer circular cylinder surface) of the light transparent cover layer is disposed inside an outer curved surface of the substrate.
According to the above arrangement, a bonding part of the light transparent cover layer and the substrate is not exposed out of the outer curved surface, so that an impact is not directly applied to the bonding part; thus, the light transparent cover layer is less likely to be peeled off.
Further, in order to achieve the aforementioned objective, the optical disk of the present invention, which includes a substrate, a light transparent cover layer bonded to the substrate with a smaller thickness than the substrate, and a first recording layer formed between the substrate and the light transparent cover layer, is characterized in that the substrate and the light transparent cover layer are formed into disks with center holes, and an inner curved surface of the light transparent cover layer is disposed outside an inner curved surface.
According to the above arrangement, a bonding part of the light transparent cover layer and the substrate is not exposed out of the inner curved surface, so that an impact is not directly applied to the bonding part; thus, the light transparent cover layer is less likely to be peeled off.
Moreover, in order to achieve the aforementioned objective, the optical disk of the present invention, which includes a substrate, a light transparent cover layer bonded to the substrate with a smaller thickness than the substrate, and a first recording layer formed between the substrate and the light transparent cover layer, is characterized in that the substrate and the light transparent cover layer are bonded via an adhesive and are welded (sealed) together.
According to the above arrangement, the substrate and the light transparent cover layer are bonded via the adhesive to each other and welded together at an outer edge and an inner edge, so as to increase strength against exfoliation caused by an impact.
Additionally, it is preferable to form projections on the welded part.
According to the above arrangement, the projections can increase the bonding strength (welding strength) between the substrate and the cover layer.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.